Neutron star mergers and supernovas are both capable of making making r-process elements. But there’s a big difference in just how much each of those options can make. Supernovas produce perhaps our moon’s worth of gold. Neutron star mergers, by contrast, make about a Jupiter-size mass of gold—thousands of times more than in a supernova—but they happen far less frequently. This allows astronomers to search for the distribution of r-process elements as a way to track their origins.
“Think of r-process [elements] as chocolate,” Ramirez-Ruiz said. A universe enriched in the r-process elements predominantly by supernovas would be like a cookie with a thin, evenly spread glaze of chocolate. By contrast, “neutron star mergers are like chocolate chip cookies,” he said. “All of the chocolate, or the rprocess, is concentrated.”
One way to assess the distribution and rate of r-process events is to look for their byproducts on Earth. Long after supernovas light up the Milky Way, the nuclei they make can coalesce onto interstellar dust grains, slip past the solar and terrestrial magnetic fields, and fall to Earth, where they should be preserved in the deep ocean. A 2016 paper in Nature that looked at radioactive iron-60 in the deep-sea crust found traces of multiple nearby supernovas in the past 10 million years. Yet those supernovas did not appear to correspond with r-process elements. When the same team looked in deep-sea crust samples for plutonium 244, an unstable r-process product that decays over time, they found very little. “Whatever site is creating these heaviest elements is not a very frequent one in our galaxy,” Metzger said.